Perpendicular magnetic recording system is adopted as a technique for increasing the magnetic recording density. A perpendicular magnetic recording medium at least comprises a non-magnetic substrate, and a magnetic recording layer formed of a hard-magnetic material. Optionally, the perpendicular magnetic recording medium may further comprise: a soft-magnetic under layer formed of a soft magnetic material and playing a role in concentrating the magnetic flux generated by a magnetic head onto the magnetic recording layer; an intermediate layer for orienting the hard-magnetic material in the magnetic recording layer in an intended direction; a protective film for protecting the surface of the magnetic recording layer; and the like.
It is proposed to use a granular magnetic material to form the magnetic recording layer in the magnetic recording medium, in order to obtain favorable magnetic properties. The granular magnetic material comprises magnetic crystal grains and non-magnetic body segregated to surround the magnetic crystal grains. Magnetic crystal grains within the granular magnetic material are magnetically separated from each other by the non-magnetic body.
For the purpose of further increasing the recording density of perpendicular magnetic recording media, an urgent need for reduction in the grain diameter of the magnetic crystal grains in the granular magnetic material arises in recent years. On the other hand, reduction in the grain diameter of the magnetic crystal grains leads to a decrease in thermal stability of the recorded magnetization (signals). Thus, the magnetic crystal grains in the granular magnetic material need to be formed of materials with higher magnetocrystalline anisotropies, in order to compensate the decrease in thermal stability due to the reduction in the grain diameter of the magnetic crystal grains. One of proposed materials having the required higher magnetocrystalline anisotropies is L10 type ordered alloys. Typical L10 type ordered alloys include FePt, CoPt, FePd, CoPd, and the like.
Japanese Patent Laid-Open No. 2005-285207 proposes a method for manufacturing a magnetic recording medium comprising a FePt magnetic thin film having a high coercive force, the method comprising the steps of: depositing FePt by a sputtering method at a substrate temperature from 650° C. to 850° C.; and applying a magnetic field from 4 kOe to 10 kOe (see PTL1). Here, Japanese Patent Laid-Open No. 2005-285207 also proposes to use a thin film of a material selected from the group consisting of MgO, ZnO, Cr and Pt, to form an intermediate layer onto which the FePt is deposited. However, the effect of the intermediate layer of the above-described material is not corroborated in Japanese Patent Laid-Open No. 2005-285207, although this document discusses the working effect in the case where a (001) monocrystalline MgO substrate is used. Further, Japanese Patent Laid-Open No. 2005-285207 does not teach or suggest to use a laminated structure consisting of different materials as the intermediate layer, at all.
Besides, several attempts have been made to improve the magnetic properties of the magnetic recording layer comprising an L10 ordered alloy with the layer formed under the magnetic recording layer. For Example, Japanese Patent Laid-Open No. 2011-165232 proposes a magnetic recording medium in which a magnetic recording layer is formed on an intermediate layer comprising MgO as a main component and one or more of additional oxides (see PTL2). In this proposal, promotion of separation between magnetic crystal grains, reduction in exchange interaction and reduction in a coercive force dispersion can be achieved by reducing a crystal grain diameter of the intermediate layer by addition of the additional oxides, and thereby forming one magnetic crystal grain in the magnetic recording layer onto one crystal grain in the intermediate layer (hereinafter, referred to as “one-to-one formation”). However, Japanese Patent Laid-Open No. 2011-165232 does not teach or suggest to use a laminated structure consisting of different materials as the intermediate layer, at all.